Although multiple-input multiple-output (MIMO) systems up to 8 or 16 antennas are supported by existing standards as so-called MIMO transmission modes, the use of very large antenna arrays (also called “massive” or “full dimension” MIMO systems) in commercial cellular systems have been proposed only recently. The 3rd Generation Partnership Project (3GPP) is currently working on the implications of supporting up to 64 transceiver units (TXRU) to serve many users simultaneously, commonly called multiuser MIMO, and/or to provide both energy efficiency and beamforming gain by creating narrow (pencil) beams when serving the scheduled users. See e.g., 3GPP TR 36.897 V13.0.0 (2015-06). In addition, 802.11 technologies aim at supporting an increasing number of antennas.
Full dimension MIMO systems are expected to increase spectral efficiency and capacity. In addition, full dimension MIMO systems are expected to provide a more uniform user experience compared to conventional MIMO systems in which the received user bit rate and/or quality of service can vary greatly depending on whether the user is located at the cell center or the cell edge. The underlying theory of full dimension MIMO systems is that under the assumption of perfect channel estimation, the vector channel of a served user grows orthogonal to other users and thereby intracell and intercell interference can be virtually eliminated.
Channel state information (CSI) acquisition presents a challenge for successful implementation of full dimension MIMO systems. In full dimension MIMO systems, the number of antennas at the access node (in the order of 100 to 1000) is assumed to be much larger than that of the served users (in the order of 10 to 20). Therefore, to limit the CSI acquisition overhead and complexity, full dimension MIMO was originally proposed for time-division duplexing (TDD) systems in which the CSI acquisition overhead is proportional to the number of antennas of co-scheduled (multi-user MIMO) users rather than the number of antennas at the access node. CSI acquisition in TDD systems is done by sending uplink pilot sequences by the users to the access node and performing channel estimation at the access node, e.g., a base station (BS) or an eNodeB (eNB) or a gNode (gNB). Pilot sequences are also often referred to as reference signals. Note that pilot sequences can be scarce resources, for example, when the coherent time/frequency resources are limited as compared with the co-scheduled multi-user (MU) MIMO users, because the length (corresponding to the number of symbols) of pilot sequences is limited by the coherence time and bandwidth of the wireless channel. In turn, the number of orthogonal pilot sequences, and thereby the number of separable users, is limited by the length of the available pilot sequences.
Because in reciprocity based full dimension MIMO systems the number of served users is much larger than that in conventional MIMO systems, pilot sequences must be reused in neighboring cells. This results in channel estimation error at the access node because of the interference generated by the users in the neighboring cells using the same or non-orthogonal pilot sequences. Such interference in full dimension MIMO systems (which, in contrast to intracell and intercell interference, is not eliminated as the number of antennas at the access node grows large) is known as reference signal interference or, in other words, pilot contamination. CSI estimation error due to pilot contamination at the access node degrades the performance in terms of actually achieved spectral efficiency, beam forming gains, receiver performance (e.g., in terms of bit error rate (BER)) and cell edge user throughput.
Summarizing, pilot sequence reuse in neighbor cells can result in pilot contamination. A technique to avoid pilot sequence reuse in neighbor cells may rely on using separation of the pilot sequences in the code domain (similar to higher frequency reuse schemes known in GSM systems). However, using separation in the code domain may be inefficient. For example, using separation in the code domain may require allocating more symbols for pilot sequence construction, which means fewer symbols can be used for user data transmission.
It is desirable to provide easy and efficient measures which may enable an accurate channel estimation.